exec-729514: Add a 'context' command

This allows us to add a magic prefix to commands. See notes in previous
commit.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
exec-729514: Refactor - unindent 'intro' command

Signed-off-by: Joe Walker <jwalker@mozilla.com>
exec-729514: Outstanding work: WARNING WILL REBASE

This branch will be rebased before it is done. You have been warned.